A proton is a subatomic particle in the center of an atom possessing a positive charge, so the plus sign designation applies. In contrast, the negative electrons outside of the nucleus of the atom possess a negative charge, and are indicated by the minus sign.
In any electrical system, electrons move from one point to another. In a transistor, electrons also move from one point to another, but they move over an ordered structure such as a crystal, where one of the electrons is missing, creating a "hole." As each electron shifts moving one way in the structure to fill the "hole", it appears the "hole" is moving the other way, contrary to the motion of the electrons. Since electrons are negatively charged, the "hole" is given a designation of a positive charge.